The present invention relates to a frequency converter which is suitable for converting high-frequency signals, such as micro-waves, into intermediate-frequency signals over a wide band.
FIG. 1 shows a conventional frequency converter. An input signal from an input terminal 11 is supplied via a directional coupler 12 to one end of a mixer diode 13. The directional coupler 12 is supplied with a local signal of a local oscillator 15 from a terminal 14 and the local signal is applied to the mixer diode 13. The other end of the mixer diode 13 is grounded via a capacitor 16 acting as a high-frequency signal path and, at the same time, it is connected to an output terminal 17.
Thus, the input signal from the terminal 11 and the local signal from the terminal 14 are provided via the directional coupler 12 to the mixer diode 13, from which a component of the frequency difference between the both signals, obtained owing to the nonlinearity of the mixer diode 13, is provided as an intermediate-frequency signal to the output terminal 17.
Since the input signal and the local signal are applied via the directional coupler 12 to the mixer diode 13, they are restricted by the frequency characteristics of the paths from the input terminal 11 to the junction of the mixer diode 13 and from the terminal 14 to the junction of the mixer diode 13 and it is difficult to operate the conventional frequency converter over a wide band.
It is also difficult to construct the prior art frequency converter so that the local oscillator 15 is capable of varying a high-frequency local signal over a wide band.